You’re hearing a male koala, although females do make barking noises when they’re excited. One study describes the koala’s grunting as “a long series of deep, snoring inhalations and belching exhalations.” Indeed.

In a book on koalas, authors Roger William Martin and Katherine Ann Handasyde describe pajama-clad campers waking park rangers, afraid of the “diabolical creature growling and roaring at them from the nearby forest.”

The call is most common during the mating season, when males are competing for partners. Males tend to sit under the canopy of trees and tilt their heads back as they call. There are six stages to koala copulation: accepting the mounting partner, a neck bite, thrusting, a pause, jerking (I’m not making this up) and disengagement. During all that excitement (which starts out sounding like a Twilight movie) the koalas are very vocal.

Females are so attracted to that sexy sound that just hearing it can make them go into oestrus. A recent study used solar-powered mobile phone stations and GPS tracking units to record 12 koalas in the wild. Once males started bellowing, females came running, and once the females are in oestrus, they start to make a barking sound.

The timbre of a koala’s bellow seems to have something to do with its size, age and androgen (a sex hormone) concentration. Older males bellow more, bigger males bellow longer, and those with more androgen have deeper bellows. One study found that when the male koala is 2-4 years older than the female, the mating is more likely to be successful, leading some to suggest that females use male calls to gage their relative sexiness. But we can’t really judge: humans do the same thing.

But strange things happen to koalas in captivity. Females, who have never been observed bellowing in the wild, will start to bellow. And in captivity, koalas have been observed displaying both heterosexual and homosexual interactions – something they’ve never been observed doing in the wild. This could happen for a number of reasons – to display dominance, relieve stress, or maintain reproductive fitness – and koalas are not the only animals to display such behavior in captivity, elephants, penguins, and many primate species do too.

The team that attached solar cell phones to koalas are continuing their research on koala language – and figuring out just what it is about that wonderful sound that makes the females all hot and bothered. Stay tuned for what they find.

Bonus fact: The first two koalas at the San Diego zoo arrived in 1925 and were named Snugglepot and Cuddlepie. Seriously.**

** A wonderful reader has informed me that Snugglepot and Cuddlepie is the name of a popular children’s book in Australia. Still hilarious names for koalas, in my opinion.